Sub-atmospheric chemical vapor deposition is used in semiconductor manufacturing to deposit thin films on substrates, for example, to deposit a silicon dioxide film on a silicon wafer. One use of sub-atmospheric CVD is in the deposition of pre-metal dielectrics (PMD). Sub-atmospheric CVD has a longer processing time than other forms of chemical vapor deposition, however, it has a much greater capability to fill trenches that are etched into wafers with very small dimensions. In these and other processes, the deposited film properties, i.e., film thickness, chemical homogeneity, and optical and mechanical properties, are important to the final device properties.
In most applications, a layer is deposited over existing features on a device. The excess coating is removed, or the variation in the coating is reduced in a subsequent chemical-mechanical deposition (CMP) step. The deposited film may also have features that are created on the film using a lithography process, followed by an etch process. Thin film deposition is an inherently complex process, thereby making it hard to simultaneously control film characteristics, such as optical and electrical properties, stresses in the film, etc., while maintaining uniform film thickness. Thin film deposition processes typically “drift” over time, causing the deposited film to deviate significantly from target values. Specifically, sub-atmospheric chemical vapor deposition introduces both radial and azimuthal thickness non-uniformity, both within and among wafers. While film thickness non-uniformity can be addressed in subsequent processing steps, the greater the deposition-induced non-uniformity, the more difficult it is to achieve within-wafer thickness uniformity in subsequent steps.
As microelectronics device feature sizes continue to shrink, it is necessary to have tighter controls in fabrication to maintain high yields. The semiconductor industry has developed run-to-run control of the various processing steps in a semiconductor fabrication process in order to reduce over process output variation from target. In run-to-run control, a product recipe with respect to a particular process is modified between machine runs so as to minimize process drift, shift, and variability. Post-process measurements are made periodically and are used along with empirical process models and drift compensation techniques to suggest new equipment settings for the next run. The development of feedback control has been largely empirical, based upon experimentally observed correlations between input and output measurements.
There has been some investigation into feedback control of plasma etch and deposition processes, both experimental and theoretical. Implementation of process control in these operations has been limited due to unavailability of suitable integrated metrology tools, limited process understanding and non-automated operational practices. Improvements in advanced process control and reduction of run-to-run variability in a sub-atmospheric chemical vapor deposition process are thus desired.